The invention relates to a method for charging and simultaneously testing the condition of a nickel-cadmium accumulator using a pulsing DC charging current of rectangular pulse shape including periods of low current intensity and periods of high current intensity, with cyclically intervening measuring phases including discharging intervals of relatively short duration, wherein the accumulator voltage is measured at the beginning of each high current-intensity period and at a preselected interval after the beginning of the respective period, and the difference of the thus measured voltage values is used for controlling the charging process. A method of this type is described in EP No. 0 034 003.
According to this publication, the change of the accumulator voltage during the charging operation is measured over a preselected time during part of a charging pulse or during part of a discharging period. The charging operation is discontinued when a parameter of the measured voltage exceeds a preselected characteristic, for instance when the voltage rises above a limit value. It is further stated that the change of the accumulator voltage is measured during a period which begins at the beginning of a charging pulse. The accumulator voltage is thus for instance measured immediately at the beginning of a charging pulse, and again two seconds later. When the accumulator is fully charged, the difference between the measured values is very important. If the measured voltage difference values are entered in a time chart, it becomes evident that the gradient of the resulting curve has a reversing point shortly before the fully charged state of the accumulator is attained, this reversing point offering itself as a criterium for discontinuing the charging operation, since this effect is independent of the temperature of the accumulator, which otherwise exerts considerable influence on the above-named voltage difference on its own.
The overcharging of a nickel-cadmium accumulator is not the only critical situation for an accumulator of this type. An accumulator consisting of mass-cells may thus not be charged in a quick-charging operation, a deep-frozen accumulator may only be charged very slowly, a completely discharged accumulator has to be re-formed prior to the charging operation proper, care has to be taken that the poles of the accumulator are properly connected, and other conditions have to be similarly observed. It is thus to be additionally observed that nickel-cadmium accumulators may assume other states in addition to the already mentioned states, which have to be taken into consideration for the charging process. The charging of a spent accumulator may thus no longer be worthwile, an accumulator in this state being better discarded. The same applies to an accumulator having a short-circuited cell. It is furthermore known that nickel-cadmium accumulators may suffer from a so-called memory effect. This means that an accumulator which has for a long time been operated under low loads or charged at weak currents is still capable of being charged to its full capacity, but is nevertheless not able to furnish a strong current under high load conditions. This state may for instance occur in the case of accumulators of radiophones which are for a long time operated in the stand-by mode and switched to transmitting operation only for short periods. This memory effect is particularly critical in the case of accumulators used for emergency current supply, for instance in the medico-technical field. It should therefore be possible to also identify this state.